WELLOG Logging While Drilling
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MONITORING WHILE DRILLING:
Often, during the course of drilling a well, accurate data is required to backup geologic structural assumptions made by drillers and geologists. A limited amount of “data” may be obtained directly from a driller or geologist based on his/her knowledge and experience having drilled other wells in the local area. Certain assumptions may be inferred from drilling speed. As different formations are drilled into, the speed of drilling varies according to the relative harness of the formation. Drillers and geologists are also aware of changes in color of the formation materials penetrated while drilling. Monitoring of this valuable information is sometimes incomplete, inaccurate or absent.
Mud logging is the process of monitoring the drilling mud that passes to the surface during the drilling process. A geologist or other person familiar with mud logging will sample the drilling mud and note the color, grain size and other attributes of the drill cuttings as defined by the objectives of the project.
In oil well drilling, certain information pertaining to hydrocarbon (oil and gas) concentration and hydrocarbon type can be obtained in almost real-time while drilling. A period of delay occurs because of the time it takes drill cuttings to circulate from the bottom of the hole to the surface. Instrumentation designed for identification of hydrocarbons i.e. hydrocarbon chromatographs are used for this purpose. Other important rock properties can also be logged. Samples can be collected for later analysis.
As noted above, a time lag occurs due to the amount of time for drilling mud to reach the surface. In air drilled holes, the cuttings represent the actual formation material drilled immediately prior to blowing the cuttings out. A cuttings trap can be used to retrieve the drill cuttings for identification.
CBM Well example:
During the process of drilling a well to produce Coal Bed Methane (natural gas), the well penetrates various formations and eventually penetrates a Coal seam known to also produce Methane gas. Drilling usually continues until the coal seam is drilled through.
Time can be saved and increased reliability achieved in determination of the final phase of drilling. Drill cuttings can be monitored for methane gas and lower natural gamma ray levels. Coal has very low (almost zero) natural gamma ray emission. Sensitive Scintillation detection equipment can be used to monitor or “Log while drilling” during this final phase of drilling. Upon completion of drilling, a thru drill pipe gamma ray – collar locator log can be obtained.
Diamond core drilling is a method often used in mineral exploration. The core drilling operation produces drill cuttings that are circulated to the surface and eventually travel to the mud pit. Circulation is suspended while the core barrel is retrieved through the drill steel. Drill cuttings may be obtained from circulation of drilling fluid. Cuttings usually settle in a shallow ditch or cuttings trap in the part of the circuit between the cuttings return at the surface of the borehole and the mud pit. Logging of representative cuttings samples by collecting, bagging, and marking depths during core retrieval provides an important backup sampling process.
Logging while Drilling:
Valuable information can be obtained from drill cuttings by logging cuttings data while drilling a well. Using sensitive equipment to analyze cuttings combined with other drilling related data can provide valuable insight about the formations being drilled and the final approach to the drilling target.